Publisher, you're not the only one, I've tried breathing pure O2 and have never felt any effects, benefical or otherwise.

I don't dispute the value of oxygen in driving out nitrogen in the case of decompression sickness. Similarly, in high altitude situations, it makes sense as you may not otherwise be getting the amount of oxygen that you need.

However, the air we breath contains about 21% oxygen on the way in and about 16-17% on the way out. If we don't take up any more oxygen than that, why bother, under normal circumstances, to put any more oxygen in the mixture. There may be some value during extreme physical activity as you are using up more O2 and so the ability to replace it more quickly may be of value. I don't know, but that makes some sense.

When it comes to diving, at least as far as I understand it, the benefit of higher oxygen levels in the gas you breath is that it reduces the amount of nitrogen.

I will, of course, bow to those who have more medical/physiological knowledge than me and would welcome more information on the science behind this "feel good" phenomenon.